1. Field of the Invention
The present invention relates to an electron microscope having a function of photographing TEM (transmission electron microscope) images of specimens and to a method of photographing such images in the microscope.
2. Description of the Related Art
Today, electron microscopes utilizing a minimum dose system (MDS) are used to observe specimens that are susceptible to electron beam damage as typified by medical and biological specimens.
The MDS is a method of minimizing the electron beam damage to the field of view to be photographed. Since the damage to the field of view to be photographed occurs mostly during a focusing operation performed prior to photography, the focusing operation is done outside the field of view in this minimum dose system.
The minimum dose system is described in further detail. This system involves three modes of operations, i.e., SEARCH mode, FOCUS mode, and PHOTO mode.
First, when the electron microscope adopting the minimum dose system is in the SEARCH mode, the second condenser lens is excited strongly as shown in the ray diagram of FIG. 1(a). Therefore, the illuminating electron beam EB is focused above the specimen and hits a wide area of the specimen. Consequently, the electric current per unit area is small. The specimen is less damaged.
The electron beam transmitted through the specimen as a result of this electron beam irradiation enters a magnifying lens system positioned behind the specimen. This lens system projects a TEM image of a wide field of view of the specimen onto a fluorescent screen. Then, an operator operates the specimen holder-moving device to bring the field of view A to be photographed into the center of the fluorescent screen, i.e., onto the optical axis, as shown in the right view of FIG. 1(a).
Then, in FOCUS mode, the second condenser lens is so excited that the illuminating electron beam EB is sharply focused onto the specimen, as shown in FIG. 1(b). The deflector is so controlled that the electron beam EB hits a region B close to the field of view A to be photographed, the field of view A having been selected in the SEARCH mode.
The operator then performs a focusing operation or stigmatic correction while watching the TEM image of the region B shown in the right view of FIG. 1(b) projected on the fluorescent screen at this time. Under this condition, the electron beam impinges on the specimen while focused. Therefore, the current per unit area is large. Damage occurs if the irradiation is done for a long time. However, the size of the irradiated area is suppressed to a minimum. Also, the illuminated position is off the field of view. Therefore, the field of view to be photographed is not damaged.
When focusing at the region B quite close to the field of view to be photographed is completed in this way, photography is performed. During the photography, deflection using the deflector is not done. As shown in FIG. 1(c), the second condenser lens is excited so that the illuminating electron beam EB is focused onto the field of view A to be photographed. A TEM image arising from the electron beam transmitted through the specimen is photographed by a photography means.
The right view of FIG. 1(c) shows an image of the area A taken by the photography means. In this case, the magnifying lens system is set at a magnification higher than in the case of FIGS. 1(a) and 1(b). Under this condition, photography is performed. At this time, the specimen is also damaged but minimally, because almost no electron beam hits the photographed region A except during photography.
As described previously, in the prior art electron microscope utilizing the minimum dose system (MDS), the excitation of the condenser lens is varied greatly when the mode of operation is switched from the SEARCH mode shown in FIG. 1(a) to the FOCUS mode shown in FIG. 1(b).
When the lens excitation varies greatly in this way, the position hit by the electron beam deviates during focusing due to a deflecting field caused by the hysteresis of the magnetic circuit or for other causes. As a result, a focused strong electron beam will hit the field of view to be photographed. This field of view should not be illuminated with the electron beam except during photography. In consequence, the field of view to be photographed will be damaged.
Also, in the SEARCH mode, the intensity of the electron beam falling on the specimen should be weakened as much as possible. In the prior art electron microscope, the electron beam cannot be darkened (i.e., cannot be spread) beyond the limit of excitation of the condenser lens.
It is an object of the present invention to provide an electron microscope which prevents the electron beam from hitting the field of view to be photographed in its FOCUS mode and can make the brightness of the beam on the specimen lower than heretofore in its SEARCH mode.
It is another object of the present invention to provide a method of photographing a TEM image in this electron microscope.
The former object is achieved in accordance with the teachings of the present invention by an electron microscope comprising: an electron gun for emitting an electron beam; a system of condenser lenses for focusing the electron beam onto a specimen; a deflection means for scanning the focused electron beam on the specimen; a system of magnifying lenses for creating a magnified image of the specimen based on the electron beam transmitted through the specimen as a result of irradiation of the electron beam; and a photography means for performing photography to take a photograph of a TEM image of the specimen magnified and focused by the system of magnifying lenses. This electron microscope is characterized in that it further includes a controller for focusing the system of condenser lenses to focus the electron beam emitted from the electron gun onto the specimen during a search operation conducted to search for a desired field of view prior to the aforementioned photography. Also, the control means controls the deflection means so that the focused electron beam scans the specimen in two dimensions.
Other objects and features of the invention will appear in the course of the description thereof, which follows.